What btu air conditioner should I get for 500 sq foot room?

I have a 500 square foot room. I also have swamp cooling (evaporative roof unit). A swamp cooler only gets your house about 15-20 degrees cooler then the outside temperature (ie: daily heat in New Mexico usually is 110 degrees outside = 95 inside). I know that without any cooling at all, I would need 15000 BTU portable air conditioner, but what about in addition to the swamp cooler? Any suggestions on the amount of btu I would need would be great. thanks in advance.